Cryogenic decontamination at the Chooz nuclear power plant: a look back at the operation
In early 2021, the Centre Nucléaire de Production d'Électricité (CNPE) de Chooz experienced a hazard: primary fluid leaked onto the vessel cover of one of its reactors. In this type of incident, the deposit remaining on the cover inevitably leads to the suspension of production. With experience of cryogenic decontamination already gained at the Flamanville power station, our teams were immediately called in and mobilized to intervene rapidly and remove the deposit. A look back at this operation.
Using cryogenics to decontaminate the reactor vessel cover
First things first... what is the nature of the deposit on the tank cover?
The water circulating in the production reactor contains boron. Boron is a kind of flour that is added to the water in the primary circuit. It is a neutrophage.
In other words, as Medhi Dujardin explains pedagogically, " It allows us to 'eat' the neutrons (produced by the fission of uranium atoms), to control and lower the chain reaction in the reactor so that the water is kept under pressure without boiling ".
When the boronated water reaches the outside of the reactor, it crystallizes and turns into a deposit, which must be "decontaminated" before the reactor can be restarted.
Dry ice blasting solution explained
The principle of dry-ice sanitization means that, through the impact and sudden cooling by CO2 (-78°C), the contracting boron layer can be easily peeled off.
The machines are equipped with accessories (angled nozzles, etc.) for various applications. After one or two passes, the cover elements regain their original appearance.
The advantages of this technique?
Compared with other solutions, dry ice generates no waste, and preserves the integrity of the equipment. " When it comes to this type of work, you have to wear velvet gloves, and dry ice lets us do just that," sums up Medhi.
A technique already tested on Flamanville
Following a boron water leak on the vessel cover of unit 2 of the Flamanville nuclear power plant at the end of 2019, we proposed dry ice blasting on the affected components. The Flamanville cover showed traces of boron which, despite several "conventional" cleanings, did not return to an acceptable condition.
Dry ice cleaning was already used on other CNPE sites and equipment (RPN chambers, pantographs, electrical cabinets, etc.). This is the first time we have duplicated the process on a vessel cover.
And the operation was a success, with the process just waiting to be deployed!
A look back
An outline of the intervention
On January 11, 2021, 14 workers and 2 site supervisors from Onet Technologies TI (Operations and Training Department) and SAO (Support to the Operational Operator) were seconded to the Chooz power station to carry out the operation.
The proposed scenario involved a 3-phase renovation: roughing, deepening and finishing.
To achieve this, our teams began by installing a work lock around the lid. This was followed by the start of cleaning operations using 3 cryogenic machines. Different workstations were set up to work in parallel on the lid and the dismantled equipment.
A total of 15 tonnes of ice were used for this operation, at a rate of 5 tonnes per phase.
A team ready to meet the challenge!
In my life as a manager, I've always found that logistics demands a high level of responsiveness. In a way, it's the DNA of our business. Dry ice has aroused a great deal of interest among our teams, with each of our employees more motivated than the last to take up this new challenge... The desire to do things right and satisfy our customers by demonstrating our know-how and capacity for innovation was our driving force! It's a great collective victory!
At the end of the service, our customer, aware that very few have this know-how, expressed his satisfaction with the organization and involvement of our teams, and our expertise in getting their site back into production quickly.
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